OAK-1643 - Implement BlobReferenceIterator optimized for Mongo

Use a while loop to handle case where node does not have binary. In such case loadBatch should try with next node document

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1583107 13f79535-47bb-0310-9956-ffa450edef68